1. Field of Invention
The present invention is in the field of needleless syringes used for intradermic, subcutaneous and intramuscular injections of liquid active principle for therapeutic use in human or veterinary medicine.
In this field, to improve the effectiveness of the injection, use is made of syringes with, at their downstream part applied to the skin or very close to the skin of the subject, an injector comprising several ducts so that the liquid that is to be injected can be distributed to several points spread over a relatively large area. This solution also has the advantage of reducing the pain and eliminating any possible superficial or subcutaneous damage that might result from an excessive amount of liquid injected at a single point.
To improve the effectiveness of the injection, the shape of the jet is also altered: the coherent distance of the jet is controlled and a solution is sought that is someway between a highly coherent jet, such as used for jet cutting and which would have very deep penetration and would cause dangerous tearing of the skin, and a jet which nebulizes the liquid and thus the fine droplets do not penetrate the skin.
2. Description of Related Art
U.S. Pat. No. 3,802,430 describes a needleless syringe in which the liquid that is to be injected is discharged by a piston pushed by gases produced by a pyrotechnic generator; that syringe has five ducts which are parallel to the axis of the syringe and have circular cross sections. U.S. Pat. No. 3,788,315 describes a syringe in which the piston discharging the liquid is pushed by the expansion of compressed gases or of a compressed spring. That syringe has six ducts of circular cross sections and the axes of which diverge slightly from the axis of the syringe. In these examples, although the liquid is spread across several points, the ducts remain fairly close together; in addition, the simplicity of the shape of these ducts shows that these ducts are not optimized for controlling the coherent length of the jet which is itself an important factor in the performance of the injector in this particular application.
More generally, the problems posed by producing an injector for a syringe are problems of mechanical strength, of performance as we have just mentioned, and of cost.
Specifically, the injector, placed at the downstream part of the syringe, must not deform under the effect of the pressure of the liquid at the time of injection: the injector has to be relatively thick, and the more widely the ducts are spread over a large area, the thicker it has to be. The problem will be that of producing ducts which in general are very fine through great thicknesses.
The performance of the injector lies in the possibility of controlling the coherent distance of the jets leaving the ducts or nozzles, for predetermined conditions of use (nature of the liquid, injection pressure), through ducts of appropriate cross sections. The purpose of this appropriate cross section is to create a field of turbulence in the flow such that, a short distance from the exit from the injector, the jet remains coherent, that is to say is fine and fast-moving enough to pierce and penetrate the skin of the subject that is to be treated, and then the jet very quickly loses its coherency: it explodes to best diffuse the active principle under the skin. The problem is that of, in a simple way, producing not only fine ducts over great thicknesses but, above all, ducts with appropriate cross sections.
Finally, the cost of manufacture becomes a very important factor in the case of mass-produced syringes, particularly for disposable syringes.